The invention relates generally to particle analyzing and/or studying apparatus having a flow cell with one or more chambers including an aperture containing means having an aperture for passage of fluid therethrough and through which particles in suspension are passed for study and more particularly to the removal of foreign matter, including debris and deposits and air bubbles, which if not so removed from the aperture and the area around it, part of its sensing zone, would effect the accuracy of the signals caused by the particles passing through the aperture. Furthermore, foreign matter from said fluid and particles may also accumulate and/or be left on or against the internal wall surfaces of said chambers and passageways leading to said aperture which if not removed could contaminate the chambers and/or serve as a source of said foreign matter which could eventually flow into the aperture sensing zone causing such accuracy problems. A potentially serious problem in this regard is a build up or presence of such foreign matter in the aperture itself, that is on its internal wall surfaces.
Since its conception in the early 1950's, the principle of particle counting and sizing invented by Wallace H. Coulter has resulted in numerous methods and flow-through apparatuses for the electronic counting, sizing, studying, and analysis of microscopic particles, which are scanned in a fluid suspension, as shown by the pioneer U.S. Pat. No. 2,656,508 to Coulter. In this prior art arrangement, a D.C. electric current flow is established between two vessels or chambers by suspending electrodes in the respective bodies or cavities of the suspension fluid. The only fluid connection between the two bodies is through an aperture; hence, an electric current flow and field are established in the aperture. The aperture and the resultant electric field in and around it constitute a sensing zone. As each particle passes through the sensing zone, for the duration of the passage, the impedance of the contents of the sensing zone will change, thereby modulating the current flow and electric field in the sensing zone, and causing the generation of a signal to be applied to a detector suitably arranged to respond to such change. (The mark "Coulter" is a registered trademark, Registration No. 995,825, of Coulter Electronics, Inc. of Hialeah, Fla.)
Systems are available for detecting and clearing the complete or substantially complete blockage of an aperture in particle study devices. One such circuit is shown and described in U.S. Pat. No. 3,259,891. This patent shows several debris clearing devices which require either complex mechanical linkages in order to mechanically remove the aperture debris or the actual removal of the aperture and/or aperture tube to manually remove the debris. The mechanical linkages are somewhat difficult to utilize and are cumbersome in operation. In the case of actually removing, cleaning and replacing an aperture tube, time is consumed which is to be avoided in operating the study devices, especially in structures with more than one aperture and furthermore this severly limits the throughput of the devices. Another debris clearing device shown in the patent employs a "burn circuit" which includes a capacitor charged to a high potential which is discharged via the electrodes creating a very high initial current flow through the aperture, thereby literally heating the contents of the aperture to explode and driving the obstruction out of the aperture. The rate of application of energy from the capacitor is not optimum or uniform and when sufficient energy is utilized to clear a blockage, it creates a serious threat of damage to the aperture material or aperture holding structure.
A second type of aperture clearing circuitry or burn circuit is shown in U.S. Pat. No. 3,963,984 which includes a pulse generator coupled to the electrode inside the aperture tube and to the electrode outside the particle tube in the fluid suspension. A pulse generator is coupled to the first and second electrodes and develops a combination of pulses having predetermined characteristics which are coupled to the electrodes and hence are coupled through the fluid contents of the aperture where they cause the fluid to vaporize and cause a microscopic explosion. Again, the force of the explosion is intended to be controlled to dislodge the debris without causing damage to the aperture or aperture structure; however, even at the optimum such an RF burst is highly energy wasteful. Furthermore, RF frequency applied to the aperture may cause a noise problem in the particle device itself. Further, it has been found that such a high frequency combination of pulses does not clean the internal surface of the aperture as completely as desired.
The particle signals may be significantly effected even though the aperture is not blocked. It is therefore useful to maintain the aperture as clean as possible without wasting energy, deteriorating the aperture structure and so that the aperture does not become increasingly smaller as a number of particle fluid samples is passed therethrough. Such cleaning is accomplished as described in the commonly assigned Canadian Pat. No. 1,163,294 of John Luscambe Martin, entitled "Aperture Cleaning System", by applying a substantially constant energy level for a predetermined period of time to the fluid in the aperture to boil the fluid therein thereby cleaning the aperture, that is cleaning the inner wall surfaces defining the aperture. The movement of fluid through the aperture may be stopped and a D.C. current applied during the rinse cycle of the devices between cycles of passing the particles in suspension through the aperture for optimum energy conservation. As described therein, the aperture will pass the particles in suspension therethrough for measuring. However, in examining white cells, the red cells are lysed and in doing so their structure is destroyed releasing their internal chemicals and protein into the suspension which then is passed through the aperture. The fluid of the suspension or electrolyte itself also contains chemicals and the protein and chemicals may build up on the internal surfaces of the aperture among other areas. As the build up increases, the size of the aperture decreases and hence the signal will vary although the same size particle has been passed therethrough each time. An ordinary rinse solution and rinsing would not eliminate this problem, hence the aforedescribed approach.
Gas bubbles, as previously indicated, are part of the foreign matter problem, and when they pass through or near the aperture and its sensing zone, they can produce erroneous particle counting and/or clogging of the aperture. One apparatus directed toward solving this problem is shown in U.S. Pat. No. 3,854,088, Counting and Analysis Apparatus for Particles Suspended in an Electrolytic Fluid, issued Dec. 10, 1974, to Gahwiler. It includes a direct-current supply electrode 11, arranged in the sample container 70, such that gas bubbles formed thereat do not reach the connection conduit 15, which is upstream of the sensing aperture 20, of the counting cell 1, and through which the sample flows. Another such apparatus is shown in U.S. Pat. No. 3,648,158, Conductivity Cell for Particle Counting, issued Mar. 7, 1972, to Parker. It includes a cell which comprises an upstream electrode 36, a sensing aperture 56, and a downstream electrode 38, within an output passageway 62, having a vent passage 72. Electrolysis resulting gas bubbles forming on the surface of the downstream electrode 38, are purged when outside air is drawn into the passageways, 61 and 62, via the vent port 74, causing purging of the system's fluid path, passageway 61 and 62, and its flow tube 16, of any remaining fluid and said gas bubbles after the completion of a counting run. Still another apparatus is shown in U.S. Pat. No. 3,781,675, Self Priming Conductivity Cell, issued Dec. 25, 1973, to Angel, wherein a priming or purging operation utilizing a bubble chamber is disclosed. Angel states:
"After an analytical run, the vent is opened to purge liquid from the cell, which eliminates conduction between the electrodes and which prepares the cell for a subsequent analytical run. (Col. 1, lines 42-52) PA0 During analysis of a particle-containing liquid, gas bubbles which can form on the electrode surfaces by reason of electrolysis or which may be present in the liquid sample can be drawn through the metering aperture to cause spurious sensing of gas bubbles as true particles and resulting in an erroneous count." PA0 "During counting operation, gas bubbles which may form on the electrode surfaces or which may be present in a sample being analyzed are collected in a position within the cell removed from the aperture to prevent their flow through the aperture during analysis. After an analytical run, in which particles in a predetermined volume of liquid are counted, the cell is operated in a priming mode to remove gas bubbles and an analyzed liquid sample from the cell, and to infuse a new liquid sample for subsequent analysis. Cross-contamination of samples is eliminated by virtue of the priming operation since an old liquid sample within the cell is replaced with a new sample prior to a subsequent counting run. PA0 During a portion of the priming operation, air is caused to flow through the aperture in a reverse direction to that of liquid flow during analysis, such air flow serving to backflush the aperture to clear any debris which can accumulate in or near the aperture during analysis." (Col. 1, line 67 to Col. 2, line 19) PA0 "Bubbles which tend to form during analytical run such as by electrolysis, or which are present in a liquid sample, rise to the top of chamber 76 away from the path of liquid flow and do not therefore interfere with the accuracy of the cell counting operation as no bubbles are drawn through the aperture 68." (Col. 5, lines 40-46) PA0 "With the plunger in its depressed position, as depicted in FIG. 6, the channels 119 are in a position straddling the lower O-ring 117 to provide a path between passages 96 and 102 to cause purging of particle-containing liquid after an analytical run and priming of the cell for a succeeding run. During the initial period of a priming operation, vent 48 is closed and a vacuum on passage 102 provided by the system pump causes liquid in passages 74 and 76 to be drawn via passage 96 through channels 116 into passages 98 and 102, and thence to the waste bottle. Any residual liquid between the aperture 68 and the input passage is also drawn out through the purging path. Air bubbles collected in the upper portion of passage 96 are also withdrawn along with the liquid. The hydraulic impedance of the purging path is lower than that of the cell couting path, and thus, during purging, liquid flow tends to bypass aperture 68 and to flow into passage 96 for removal from the cell. PA0 After purging of the liquid from the cell, vent 48 is opened and, with plunger 100 remaining depressed, air is drawn into the vent passage and through aperture 68 in a direction opposite to that of liquid flow during a counting run, and thence via passages 76, 96, channels 116 and passage 102 into the waste bottle. Air and bubbles within the flow tube 16 are also drawn into the waste bottle during this operating state. The reverse flow of air through aperture 68 causes backflushing of debris which can accumulate in the aperture, and, thus, the invention provides not only purging of the cell for a subsequent analytical run but also automatic cleaning of the aperture between runs." (Col. 5, line 53 to Col. 6, line 17)
The patent's stated object is "to provide an improved conductivity cell in which bubbles within the cell do not materially affect system performance." Angel goes on to further explain that:
Angel describes the operation of the bubble chamber as follows:
He further describes in detail the operation of the preferred embodiment as follows: